1. Field of the Invention
The present invention is related generally to a method for enveloping an electronic component that has a generally planar form with two base surfaces lying opposite one another in which bond wires proceed from one of the two base surfaces and, in particular to a method in which a flowable gel is applied to the base surface of the electronic component to form an intermediate layer which is then surrounded by an enveloping compound.
2. Description of the Related Art
An enveloping technique for electronic components is disclosed in published European patent application 88 11 3887.9.
Sensitive components such as complex integrated circuits are usually enveloped with a plastic compound to protect them against detrimental environmental effects. Such plastic compounds, also referred to as enveloping compounds, have been previously developed and provide a reliable shield for the component from atmospheric influences, but these enveloping compounds are not particularly well adapted to the thermal expansion properties of the chip material so that unacceptably high mechanical stresses can be placed on the component, particularly when great fluctuations in operating temperatures occur. Therefore, in some instances, the chip surface has been covered with an elastic and compressible intermediate layer which compensates for differences in thermal expansion and, thus, enables a wider selection of the various enveloping plastics to be used.
The enveloping technique disclosed in European patent application 88 11 3887.9 proposes the use of a variety of materials and application methods for the intermediate layer. For example, the manual application of ultraviolet-curable silicone rubber or the dosed application of a polyether urethane is proposed. To provide a surface covering of a dosed material, the intermediate coating compound is forced through a hollow needle that is guided in a meandering path over the chip, or it is forced through a slot-shaped nozzle.
In practice, the substances and application techniques disclosed in published European patent application 88 11 3887.9 provide definitely satisfactory results for small-area chips. However, for chip areas of approximately one square centimeter or more, the known techniques do not succeed in reliably covering the edge regions of the chip without further ado, these edge regions being inherently in need of special protection against mechanical influences due to the sensitive bond wire connections. The principle reason for the lack of adequate covering in the edge regions of the chips is that the applied plastic material contracts to a drop shape relatively quickly due to surface tension.
When it is important that the external surface of the enveloped component is flat, a further disadvantage is present using the known practice since the intermediate layer has contracted or flowed together with sufficient thickness toward the middle of the chip that in certain circumstances the middle of the chip may no longer be coated by the enveloping compound and the intermediate layer may emerge from the enveloping compound to a more or less pronounced degree. In this case, the component is no longer completely encapsulated and, moreover, does not have the prescribed shape.
Theoretically, there are a number of possibilities for preventing or eliminating the formation of drops. The intermediate layer may be realized in an injection molding process. Such technique provides a well-defined layer profile and can also be automated. However, injection molded processes require injection molding tools that are constructed in relatively complicated ways and, in addition, the process is rather time consuming and does not allow the layer to be limited to only the chip surface without further ado. Thus, the entire bond wires including their contacts together with the terminal legs are coated by the intermediate layer material, which must be strictly avoided in certain types of chips.
It is also conceivable to drip the intermediate layer material onto the center of the area to be coated and then distribute the material by rotating the component, as in German patent document DE AS 26 37 105. Finally, there is an alternative process disclosed in German published application 34 42 131 in which a sealable substance is poured over the chip and then the material mass is pressed into the desired shape with a foil that is pressed onto the mass under elevated temperatures. It is obvious that the two foregoing methods are so involved that they actually do not come into consideration for mass production.